1. Field of the Invention
The invention concerns an apparatus making it possible to measure long distance the deformations of an object subjected to stresses, the object being, for example, an inertia wheel revolving at high speed in an vacuum chamber.
2. Description of the Related Art
Such an apparatus is designed to study the behavior of parts subjected to stresses, whether these parts be revolving parts or non-revolving parts subjected, for example, to alternate vibrations or displacements, whenever it is desired to know the maximum amplitudes concerning these parts.
In particular, the apparatus of the invention may be used to study the behavior of the rotors of kinetic energy storage systems, said rotors being used on space instruments, such as man-made satellites, so as to provide the devices put on board these instruments with the energy required for their functioning when these instruments or satellites are in a shadow zone. These kinetic energy storage systems have been developed with the aim of replacing electric batteries which used to constitute the main power source of satellites and represented a considerable on board weight.
The rotor or inertia wheel of such a kinetic energy storage system is composed of several functional elements whose effective element is a wheel rim driven at a high rotation speed (about 30,000 rpm for a diameter of 600 mm) so as to obtain a high kinetic energy per unit of mass. This wheel rim is connected by arms to a central hub supported by magnetic suspension. In addition, static and dynamic balancing means are provided.
Having regard to the extremely high rotation speed of the rotor under vacuum, the centrifugal stresses exerted on the rotor are considerable. A knowledge of the behavior of rotors subjected to such stresses constitutes an important element as regards the design of these rotors. In particular, it is desirable to be able to accurately measure deformations, such as the elongation and distension of rotors revolving at high speed.
So as to carry out such measurements, there currently exist several installations making it possible to restore on the ground the operating conditions of the rotor in space by placing this rotor in a vacuum chamber and by making it revolve at high speed inside this chamber.
Some of these apparatus make use of eddy current displacement transducers. However, these transducers cannot be used when the rotor does not comprise any conductive surface or when the environment of the rotor is a conductive medium. Moreover, the shape, dimensions and the nature of the materials constituting the rotor affect the impedance of the coil of the transducer and, as a result, the quality of the measurement. Furthermore, given the fact that the measurement carried out is an average of the respective elongations of the rotor, it lacks accuracy to a large extend.
In addition, the use of eddy current displacement transducers also results in measurement errors owing to thermal derivatives, which are made up for on the transducer but not on the rotor. Accordingly, the magnetic permeability of the rotor varies with the temperature, the same applying to its resistivity when the thickness of the rotor is less than the penetration depth of eddy currents. Measurement errors also occur when the bending radii of the rotors are too large with respect to the size of the transducer or when the surfaces of the rotors are smaller than those of the transducer. All these causes of errors make it necessary to calibrate the transducer according to the temperature of the rotor. Accordingly, the device needs to be calibrated for each form of rotor and each type of material. This calibration requires several adjustment parameters in order to use these transducers, this implementation proving to be particularly delicate.
Some of known measurement apparatus include in particular optical installations functioning on the basis of a black/white contrast measurement. As with those apparatus using eddy current transducers, this measurement is an average and results in a considerable loss of precision. Moreover, these apparatus present difficulties in sue and involves problems regarding calibration and lighting. Furthermore, they are relatively expensive to implement.
Other known apparatus do not make it possible to carry out a deformation measurement but merely display these deformations by means, for example, of a video system. However, these apparatus have the drawback of being limited by virtue of the number of images emitted during image recording. Furthermore, no measurement is made and no item of data is recorded.
So as to carry out measurements of the deformation of a rotor revolving at high speed in a vacuum chamber, it is possible to use other methods, such as interferometry using the working difference between two optical path lengths or laser range finding. However, these methods also present difficulties as regards implementation and application, as, generally speaking, they are delicate to handle.